1. Field of the Invention
This invention relates to RF (radio frequency) transponder systems, and more particularly to transponder systems with a self-resonant antenna implemented on one side of a substrate that can be affixed to an object to be identified.
2. Description of the Related Art
The development of machine readable identification tags for applications such as airport luggage systems and package identification has allowed for more efficient and cost effective identification. Bar-coded labels are most popularly used for this purpose. The labels are flexible and have an adhesive coating on their rear surface that allow them to be applied to objects of many different sizes and shapes. However, bar-code systems have several undesirable limitations. They can be read only along a line-of-sight, require the reader to be positioned relatively close to the label being read, can produce false readings in the case of very dirty or obscure labels, and also require that the reader be properly oriented relative to the label.
RF transponders have also been developed that provide an identification code, or at least an indication of the presence of the transponder. An interrogator transmits an RF signal that is picked up by the transponder antenna. The antenna either powers a circuit that is included in the transponder and retransmits an identification code, or couples back to the interrogator in the case of an "I am here" system in which only the presence of the transponder, not its identification, is sensed. Transponders provide an identification mechanism that can be read even when the transponder is not within the sight of the interrogator, operate at longer ranges than bar code systems, are not subject to errors because of dirt accumulation, and do not require any particular physical orientation between the transponder and the interrogator.
Most transponders consist of a coil of wire that is stuck together during manufacture to form a relatively stiff planar body. A small printed circuit board that includes an IC chip for the identification code, and also a chip capacitor, are glued to the coil, which is then typically laminated between two sheets of plastic to produce a product with an appearance like a credit card. The capacitor is selected so that, together with the coil inductance, it forms a tuned circuit that resonates at the interrogator frequency to enhance the coupling of energy into the transponder circuit. More recently, transponders have been developed in which a capacitor is integrated into the IC chip, rather than as a discrete device. A typical transponder system that transmits an identification code in response to an interrogation signal at its tuned frequency is described in U.S. Pat. No. 4,730,188 to Milheiser.
In the case of an "I am here" transponder system, which is useful for example in electronic article surveillance systems, an aluminum antenna coil has been formed on opposite sides of a dielectric sheet by stamping or embossing, the use of metalized thin films or conductive paints, or bonding pre-cut spiral patterns onto the sheet. The two halves of the coil on opposite sides of the sheet are aligned with each other, producing a self-capacitance that results in resonance at the desired frequency. Such a system is described in U.S. Pat. No. 4,598,276 to Tait. The antenna coil's self-capacitance eliminates the need for a discrete capacitor, or reduces the size of any additional capacitor that may be required.
While transponder systems have advantages over barcode systems in their ability to read an identification code from a distance, a typical transponder is considerably more expensive than a bar-code label, and available transponders cannot easily be affixed to a wide variety of objects with different sizes and shapes.